The researchers identified DNA from a variety of mammals, including woolly mammoths, woolly rhinoceroses, cave bears and cave hyenas. Mixed in with this animal DNA were small traces of human DNA: The researchers found Neanderthal DNA in four caves, and Denisovan DNA in Denisova Cave.

"The fact that sediment can indeed preserve DNA from extinct humans that lived there thousands of years ago is a pretty amazing finding," Meyer said.

In addition, at each of the two sites where the researchers did not discover DNA from ancient humans, they had only a few samples to analyze, Meyer noted. "Maybe if we looked at more samples from each site, we'd find Neanderthal or Denisovan DNA as well," he said.